This invention relates in general to fuel cells, and more particularly to a system for controlling hot spots that tend to occur on fuel cell membrane electrode assemblies.
In recent years, nearly all electronic devices have been reduced in size and made lightweight, in particular portable electronic devices. This advancement has been made possible, in part, by the development of new battery chemistries such as nickel-metal hydride, lithium ion, zinc-air, and lithium polymer that enable larger amounts of power to be packaged in a smaller container. These secondary or rechargeable batteries need to be recharged upon depletion of their electrical capacity. This is typically performed by connecting the battery to a battery charger that converts alternating current to a low level direct current of 2-12 volts. The charging cycle typically lasts a minimum of 1-2 hours, and more commonly 4-14 hours. Although the new batteries are a tremendous advancement over the previous generations of batteries, they still suffer from the need for sophisticated charging regimens and the slow charging rates.
Fuel cells are expected to be the next major source of energy for portable electronic products. Simply put, fuel cells catalytically convert a hydrogen molecule to hydrogen ions and electrons, and then extract the electrons through a membrane as electrical power, while oxidizing the hydrogen ions to H2O and extracting the byproduct water. The tremendous advantage of fuel cells is the potential ability to provide significantly larger amounts of power in a small package, as compared to a battery. Their potential ability to provide long talk-times and standby times in portable electronic device applications are driving miniaturization of fuel cell technologies. The polymer electrolyte membrane (PEM) based air-breathing, dead-ended fuel cells are ideally suited for powering portable communication devices. One of the key operating challenges in a small dead-ended fuel cell system is temperature regulation at different points on the cells. Since these fuel cells do not have forced gas circulation or external membrane water management systems, the distribution of fuel gas and water over different areas of the membrane electrode assembly (MEA) of the fuel cell will be non-uniform. This non-uniform distribution has the potential to create significant hot spots which can destroy the MEA and hence the performance of the fuel cell.
In a dead-ended air-breathing hydrogen/air fuel cell, the electrolyte membrane would have a tendency to dehydrate when it is operated at a relatively high current. As the membrane dries, the internal resistance of the cell increases, and the power output of the cell is substantially reduced. Undesirably, this drying out can progressively march across the PEM until the fuel cell fails completely. The increase in internal resistance produces I2R heating which develops into xe2x80x9chot spotsxe2x80x9d. Though prior art technologies exist to control some sources of hot spots, there is no practical prior art technique that will significantly eliminate the probability of encountering hot spot conditions. In addition, the prior art methods use active systems, that require external components, which are not practical for a portable fuel cell based power source. Most of the prior art methods focus on membrane hydration and water management methods to prevent the creation of hot spots. For example, U.S. Pat. Nos. 5,858,567, 6,156,184, and 6,207,312 teach various techniques for membrane hydration. These methods are based on an approach which attempts to keep the entire electrolyte membrane hydrated at high enough level to survive the highest level of current density without drying out the membrane. In addition, hot spots can be caused by factors other than electrolyte membrane dehydration such as poor distribution of fuel gases. The prior art methods are cumbersome and are not fail-proof; they can fail for a variety of reasons including non-uniformity of membrane material, aging of the materials and localized variations in concentration of fuel gas. In addition, maintaining a high level of hydration to keep the entire electrolyte membrane fully hydrated can cause flooding on the cathode side of the fuel cell. Therefore, a better approach is needed for controlling hot spots in fuel cell systems.